Vanadium-aluminum-titanium master alloys

ABSTRACT

THIS INVENTION RELATES TO IMPROVED VANADIUM-ALUMINUMTITANIUM MASTER ALLOYS FREE OF SLAG VOIDS AND GROSS (MACROSCOPIC) NITRIDE INCLUSIONS, BY REASON OF WHICH THE MASTER ALLOYS ARE PARTICULARLY USEFUL INPROVIDING TITANIUM BASE ALLOYS OF GREATER SOUNDNESS. THIS INVENTION ALSO CONTEMPLATES A NOVEL PROCESS INVOLVING ALUMINOTHERMIC COREDUCTION OF VANDAIUM PENTOXIDE AND TITANIUM DIOXIDE TO OBTAIN SUCH IMPROVED VANADIUM-ALUMINUM-TITANIUM MASTER ALLOYS.

United States Patent US. Cl. 75-134 V 13 Claims ABSTRACT OF THE DISCLOSURE This invention relates to improved vanadium-aluminumtitanium master alloys free of slag voids and gross (macroscopic) nitride inclusions, by reason of which the master alloys are particularly useful in providing titanium base alloys of greater soundness. This invention also contemplates a novel process involving aluminothermic coreduction of vanadium pentoxide and titanium dioxide to obtain such improved vanadium-aluminum-titanium master alloys.

This application is a continuation-in-part of application Ser. No. 617,031, filed Feb. 20, 1967, now abandoned.

BACKGROUND OF INVENTION Titanium base alloys, such as those which contain 4% vanadium and 6% aluminum, are used in the blades of jet propulsion engines for aircraft by reason of their high strength, both hot and cold, and their resistance to oxidation. Such titanium base alloys are generally prepared from master alloys, particularly the 40% vanadium-60% aluminum master alloy. In order to minimize the quantity of contaminants, interstitials and inclusions in the ultimate titanium base alloy, considerable effort has been given to producing vanadium-aluminum master alloys of great purity.

Vanadium-aluminum master alloys, such as the 40% vanadium-60% aluminum alloy, are generally prepared by aluminothermic reduction of vanadium pentoxide in the presence of a molten flux. During the reaction, vanadium metal is ,produced which alloys with the aluminum present in excess of that required for reduction of vanadium pentoxide. Alumina, which is formed, enters the molten flux which floats on top of the alloy.

One of the problems in the production of such vanadium-aluminum master alloys is slag voids and the formation of nitrides, which appear generally as gross (macroscopic) inclusions. These inclusions, which can be detected only with ditficulty, can easily go undetected even if the alloys are subjected to rigid inspection procedures. If these nitrides go undetected and are carried into titanium base alloys, blades of aircraft jet propulsion'engines made therefrom can fracture in use, with potential loss of life and equipment due to engine failure.

DESCRIPTION OF INVENTION It is, therefore, an object of the present invention to provide a process for the preparation of master alloys containing vanadium and aluminum which are free of slag voids and gross inclusions of nitrides, and from which titanium base alloys of improved physical properties and greater soundness may be prepared.

Another object of this invention is to provide novel master alloys containing vanadium and aluminum wherein nitrides, if present, are there in only extremely minute quantity, and are widely dispersed and microscopic in size,

in which form they do not adversely atfect the physical properties of titanium base alloys prepared from such master alloys.

Still another object of this invention is a process for preparing such master alloys which may be carried out in a relatively simple manner and without requiring the use of expensive apparatus or materials While resulting in master alloys of excellent quality at high yields.

These and other objects of this invention will become further apparent from the following specification and appended claims.

According to this invention there is provided a process for preparing a vanadium-aluminum-titanium master alloy which is substantially free of slag voids and nitride inclusions having an average diameter greater than about 0.01" which comprises reacting a mixture of vanadium pentoxide and titanium dioxide with aluminum above the melting point of aluminum whereby these oxides are reduced to metallic vanadium and titanium which alloy with aluminum, and alumina is formed. In carrying out the reaction, the respective amounts of vanadium pentoxide, titanium dioxide and aluminum are proportioned so as to provide a vanadium-aluminum-titanium alloy containing from 40 to 55% of vanadium, from to 40% of aluminum, and from 0.5 to 5% of titanium.

As stated above vanadium-aluminum master alloys generally have been prepared by aluminothermic reduction of vanadium pentoxide. The amount of aluminum employed is in excess of that required for reduction of vanadium pentoxide and is available for alloying with vanadium metal produced by the reaction. Unfortunately, ingots of the alloy so produced frequently contain slag voids and gross nitride inclusions. These impurities, which are difiicult to detect, find their way into titanium base alloys prepared from such master alloys, and adversely aifected the physical properties of the titanium base alloys.

According to this invention, it was discovered that the above-described problems could be overcome if the aluminothermic reduction of vanadium was carried out in the presence of a small amount of titanium dioxide. The function played by the titanium dioxide is not fully understood. Chemical analysis of ingots so produced show that even if they contain approximately the same amount of nitrogen as ingots prepared by the aluminothermic reduction of vanadium pentoxide in the absence of titanium dioxide the ingots are free of gross nitride inclusions. However, a significant difference was noted in the alloys produced by the respective processes. Where titanium dioxide was not present during reduction, the ingots frequently contained undesirable gross (macroscopic) nitride inclusions. In addition, there were numerous voids in the ingots, some of which contained slag. On the other hand, where titanium dioxide was present and was coreduced with the vanadium dioxide in the presence of excess aluminum, a master alloy of vanadium, aluminum and titanium was obtained which was free of gross nitride inclusions and substantially free of undesirable voidsv Any nitrides found to be present were microscopic in size and were uniformly dispersed throughout the alloy.

Apparently when the nitride inclusions are microscopic, i.e. have an average diameter not exceeding about 0.01", and are rather widely dispersed in the master alloy, although such inclusions appear in titanium base and other alloys prepared therefrom, the physical properties of the latter alloys are not adversely affected to any significant degree. Thus, this invention makes possible the production of nominal 40% vanadium-60% aluminum master alloys containing a small amount of titanium metal which are particularly suitable for producing titanium base alloys, such as those containing 4% vanadium and 6% aluminum, having considerably improved properties. Parts for aircraft jet propulsion engines fabricated from such titanium base alloys are less likely to fail during use. This improved reliability of aircraft engine parts is of great importance to the aircraft industry.

The invention is described in greater detail hereinafter particularly with reference to preparing a nominal 40% vanadium-60% aluminum master alloy containing a small amount of titanium. However, it is to be understood that the invention is applicable to preparation of other master alloy compositions wherein the metals vanadium, aluminum and titanium are present in the above-stated ranges.

Any known type of reaction vessel may be employed in producing the master alloys of this invention. For example, the reaction may be caused to take place in a simple copper pot or crucible. Since the reduction reaction is exothermic, the employment of a reaction vessel having a water jacket to control temperature is particularly desirable. In addition, inasmuch as the reaction produces two separate layers, i.e. an alloy layer covered by a layer of molten slag-containing flux, it may be desirable to employ a reaction vessel having a tap hole toward the bottom to aid in separation of alloy from the flux. If desired, the reaction vessel may be so constructed as to permit carrying out the aluminothermic reaction in an atmosphere of an inert gas, such as argon. A preferred type of reaction vessel is a water-cooled copper vessel of the type described in Metallothermic Reduction of Oxides in Water-cooled Copper Furnaces, by F. H. Perfect, Transactions of the Metallurgical Society of AIME, vol. 239, Sgst 67, pp. 1282-1286.

In carrying out the process of this invention the vanadium pentoxide, titanium dioxide and aluminum may be reduced to relatively small grain size and intimately mixed so that the reaction will occur very rapidly and uniformly throughout the charge once it is ignited. More aluminum is added than is necessary to react with the metal oxides in order to produce an alloy of the metals vanadium, aluminum and titanium.

Ignition of the reaction mixture may be effected by heating the charge above the melting point of the alumi- 1nEm by an electric arc, gas burners, hot metal bar or the To be successful, substantially all of the reaction products resulting from ignition of the charge must be melted and remain in the molten state long enough to permit separation of the alloy from the slag, i.e. alumina. Since the separation is by Stratification due to gravity, it is necessary that the molten materials have substantial fluidity. Fluidity of the alumina slag may be obtained by inclusion in the charge of certain inorganic materials which act as a flux to lower the viscosity of the slag. Typical of these materials are lime and fluorspar, which form a molten flux at reaction temperatures for absorption of the alumina slag. These materials generally remain unaffected by the reduction reaction.

While the process of the present invention may be carried out with chemically pure vanadium pentoxide, a particular advantage of the invention lies in its ability to produce improved vanadiumaluminum-titanium master alloys from commercial grade vanadium pentoxide, and even from less pure technical grades of such oxide.

Commercial grade V is usually described in the trade as fused black vanadium oxide flake, and analyzes 95% or more of V 0 the balance being largely the oxides of sodium, potassium and silicon, with minor amounts of oxides of iron and other impurities, including sulfur and phosphorus.

Technical grade V 0 contains essentially the same constituents as commercial grade V 0 except that it comprises only about 87-93% V 0 and is much higher in the oxides of sodium and potassium.

Commercial grade V 0 is a preferred V O -containing material for use in the present invention, since improved master alloys can readily be obtained without the high 4 volume of smoke that is produced when technical grade V 0 is used.

Advantageously, the present process does not require the use of chemically pure titanium dioxide. Thus, although pigment grade, commercially pure titanium dioxide which analyzes 99% plus TiO is preferred, less pure Tio -containing material, such as native rutile, which analyzes about 96 Ti0 and contains as impurities minor amounts of the oxides of Fe, Si, Zr, Cr, Al and Ca, as well as S and P, can also be employed. Since the amount of TiO -containing material employed is relatively small, preferably commercially pure TiO is used to enhance the overall purity of the resulting alloy.

The aluminum should be of the highest purity which is commercially available. Chopped aluminum wire (a relatively pure conductor material) containing less than 0.005% boron can be employed. However, virgin aluminum powder which analyzes in excess of 99% of aluminum is the preferred reducing agent and alloy addition according to the present process.

Since the metal oxide and aluminum reactants may vary in purity, the proportions thereof to provide an alloy of a given composition will vary accordingly. For this reason, in this description and appended claims, the respective amounts of reactants are expressed in terms of the composition of the desired alloy. As stated hereinabove, the amounts of these reactants should be so proportioned as to provide a master alloy containing from 40 to 55% of vanadium, from 60 to 45% of aluminum, and from 0.5 to 5% of titanium. When the analyses of the various reactants are known, the proportions thereof which must be employed to provide a particular alloy composition can be readily determined. Preferably, the proportions of these reactants are such as to provide an alloy containing 40 to 43% of vanadium, 60 to 50% of aluminum, and from 1 to 5% of titanium.

During the reaction alumina slag is produced. As stated above, in order to aid in separating the slag from the alloy, the reaction is carried out in the presence of a molten flux which dilutes the alumina slag and renders it in a more fluid form.

The molten flux which may be employed in the process of the present invention may comprise one or more inorganic materials having a melting point below the temperature at which the vanadium pentoxide and titanium dioxide react with the aluminum. This flux must be capable of diluting the alumina slag formed by the reaction to produce a less viscous slag which easily separates from the alloy. The readily available fluorides and chlorides of such metals as Ca, Na, Al and K, alone or in combination with other inorganic materials are particularly suitable for forming alumina-absorbing fluxes. A particularly preferred flux is one formed from lime and fluorspar whereln the weight ratio of the former to the latter is from about .5:1 to 2:1.

The amount of flux-forming constituents employed should be suflicient to provide an amount of molten capable of diluting the alumina slag that is formed during the reduction of the oxides of vanadium and titanium to provide a less viscous slag which is readily separated from the metal. Preferably, an excess of flux over that needed to obtain the desired viscosity reduction is used. This excess may generally be from 0.52 times the weight of alumina slag formed in the process.

An advantageous feature of this invention is that scrap cleanings from previous production runs can be recycled and absorbed into the melt. Thus, there can be added to the molten aluminum prior to addition of metal oxide a recycled nominal 40% vanadium-60% aluminum alloy. The amount of recycle alloy which can be used Will depend upon its purity and composition. Ordinarily recycle alloy can be present in an amount of up to 45% of the combined weight of metal oxides and aluminum fed to the reactor. The greater the purity and the closer the recycle alloy is in composition to the desired vanadiumaluminum-titanium alloy, the more of such recycle alloy may be used.

The resulting vanadium-aluminum-titanium alloy contains less than about 0.007% nitrogen, and any nitrides present are microscopic in size, generally less than 0.01" in average diameter, and are widely dispersed throughout the alloy.

The invention is particularly valuable in producing nominal 40% vanadium-60% aluminum master alloy containing a small amount of titanium. Such master alloy can be used to make various titanium base alloys, including the 4% vanadium, 6% aluminum alloy.

In making titanium base alloys utilizing the master alloys of this invention, the titanium is preferably melted by the consumable electric arc process in a water-cooled copper crucible in a vacuum or an inert atmosphere, such as an atmosphere of argon, and the master alloy is added to the melt. The titanium may be either commercial titanium or high purity titanium. The amount of titanium added should be such as to provide an allo of the desired composition.

The .following examples illustrate the use of the abovedescribed process in producing novel V-Al-Ti master alloys according to the present invention. It should be understood that the examples are given by way of illustration rather than by way of limitation and that many changes may be made therein without departing in any way from the scope and spirit of the invention.

EXAMPLE I 93 lbs. of coarse aluminum powder and 35 lbs. of 40% vanadium-60% aluminum alloy scrap (less than 1% impurities, including Si, Fe, O, N) were charged into a water-cooled copper furnace. 66 lbs. of V O -containing material (analyzing about 98% V the balance largely K 0, Na O and SiO with minor amounts of oxides of Fe and other impurities including S and P) and lbs. ofTiO -containing material (analyzing about 96% TiO the balance largely oxides of Fe, Si, Zr, Cr and A1, with minor amounts of CaO, P and S) were pulverized and intimately mixed with the aluminum and alloy in the furmice. lbs. of pulverized lime, 15 lbs. of acid grade fiuorspar and lbs. of sodium chlorate were added to the mixture in the furnace. The mixture was ignited and the heat of reaction quickly raised the temperature of the mass to about 2400" C.

Aluminum oxide formed during the reaction between the aluminum and metal oxides separated from the metal and was diluted by the molten flux formed from the lime and fiuorspar. The slag of reduced viscosity formed a layer which floated on top of the alloy.

Thereafter, the alloy was allowed to freeze in the copper furnace. After the metal had solidified, the resulting ingot was cropped to remove slag. The ingot obtained weighed 118 lbs. and had the following analysis:

Metallurgical studies revealed that the ingot was free of gross inclusions of nitrides. In addition, the ingot was sound and substantially free of voids.

EXAMPLE II The procedure of Example I was repeated using the charge as therein described, with the exception that the amount of TiO -containing material was reduced to 5 lbs.

6 The ingot obtained weighed 112 lbs. and had the following analysis:

Constituent: Weight percent Vanadium 41.91 Aluminum 55.78 Titanium 1.33

Silicon 0.34

Iron 0.24

Oxygen 0.071 Nitrogen 0.005

Metallurgical studies showed that the ingot was sound and substantially free of voids. Nitride inclusions, if any, were widely dispersed and could not be seen with the naked eye.

' EXAMPLE [[II The procedure of Example II was repeated and an ingot weighing 114 lbs. having the following analysis was obtained:

Constituent: Weight percent Vanadium 41.67 Aluminum 53.52 Titanium 3.40 Silicon 0.42 Iron 0.44 Oxygen 0.071 Nitrogen 0.003

Microscopic examinations of the ingot revealed that it was sound and substantially free of voids.

EXAMPLE IV The procedure of Example I was again repeated using the charge described therein, with the exception that the amount of TiO -cOntaining material was reduced to 2 lbs. The ingot obtained weighed 112 lbs. and had the following analysis:

Constituent: Weight percent Vanadium 42.35 Aluminum 55.95 Titanium 0.76 Silicon 0.38 Iron 0.50 Oxygen 0.038 Nitrogen 0.003

The ingot had some sponge holes; however, it was free of gross inclusions of nitrides.

While the invention has been described with specific embodiments, it is to be understood that it is not limited thereto, but is to be construed broadly and limited solely by the scope of the appended claims.

What is claimed is:

1. A process for preparing a vanadium-aluminumtitanium alloy which is substantially free of nitride inclusions having an average diameter greater than about 0.01 which comprises, reacting a mixture of vanadium pentoxide and titanium dioxide with aluminum above the melting point of aluminum, whereby said oxides are reduced to metallic vanadium and titanium which alloy with aluminum, and alumina slag is formed, the amount of vanadium pentoxide, titanium dioxide and aluminum being proportioned so as to provide a vanadium-aluminum-titanium alloy containing from 40 to 55% of vanadium, from 60 to 45% of aluminum, and from 0.5 to 5% of titanium.

2. The process according to claim 1 in which the amount of vanadium pentoxide, titanium dioxide and aluminum are proportioned to provide a vanadiumaluminum-titanium alloy containing from 40 to 43% of vanadium, from 60 to 50% of aluminum, and from 1 to 5% of titanium.

B. The process according to claim 1 in which said reaction is carried out in the presence of up to 45%, by

Weight, of an alloy of vanadium and aluminum, based on the combined weight of said mixture of oxides and aluminum.

4. The process according to claim 1 in which said reaction is carried out in the presence of a molten flux.

5. The process according to claim 4 in which said flux comprises a mixture of lime and fiuorspar.

6. A process for preparing a vanadium-aluminum-titanium alloy which is substantially free of nitride inclusions having an average diameter greater than about 0.01" which comprises, reacting a mixture of vanadium pentoxide and titanium dioxide with aluminum above the melting point of aluminum, whereby said oxides are reduced to metallic vanadium. and titanium which alloy with aluminum, and alumina slag is formed, said reaction being carried out in the presence of an amount of a molten flux comprising lime and fluorspar sufficient to reduce the viscosity of said alumina slag, the amount of vanadium pentoxide, titanium dioxide and aluminum being proportioned so as to provide a vanadium-aluminum-titanium alloy containing from 40 to 43% of vanadium, from 60 to 50% of aluminum, and from 1 to 5% of titanium.

7. The process according to claim 6 in which said reaction is carried out in the presence of up to 45 by weight, of an alloy of vanadium and aluminum, based on the combined weight of said mixture of oxides and aluminum.

8. A vanadium-aluminum-titanium alloy containing from 40 to 55% of vanadium, from 60 to 45% of aluminum, and from 0.5 to 5% of titanium, said alloy containing not more than about 0.007% by weight, of nitrogen, and being substantially free of nitride inclusions greater than 0.01 in average diameter.

9. A vanadium-aluminum-titanium alloy containing from 40 to 43% of vanadium, from 60 to 50% of aluminum, and from 1 to 5% of titanium, said alloy containing not more than 0.007%, by weight, of nitrogen, and being substantially free of nitride inclusions greater than 0.01" in average diameter.

10. A process for preparing a vanadium-aluminum-titanium alloy which contains not more than about 0.007% of nitrogen and is substantially free of nitride inclusions greater than 0.01" in average diameter, which comprises, reacting a mixture of commercial grade vanadium pentoxide and commercially pure titanium dioxide with an aluminum-containing material analyzing in excess of 99%, by weight, of aluminum above the melting point of the aluminum, whereby said oxides are reduced to metallic vanadium and titanium which alloy with aluminum, and alumina slag is formed, said reaction being carried out in the presence of an amount of a molten flux sufficient to reduce the viscosity of said alumina slag, the amount of vanadium pentoxide, titanium dioxide and aluminum being proportioned so as to provide a vanadium-aluminum-titanium alloy containing from to of vanadium, from to 45% of aluminum, and from 0.5 to 5% titanium.

11. The process according to claim 10 in which the amount of vanadium pentoxide, titanium dioxide and aluminum are proportioned to provide a vanadium-aluminum-titanium alloy containing from 40 to 43% of vanadium, from 60 to 50% of aluminum, and from 1 to 5% of titanium.

12. The process according to claim 10 in which said reaction is carried out in the presence of up to 45 by weight, of an alloy of vanadium and aluminum, based on the combined Weight of said mixture of oxides and aluminum.

13. The process according to claim 12 in which said flux comprises a mixture of lime and fiuorspar.

References Cited UNITED STATES PATENTS 2,578,098 12/1951 Southard 138 2,880,087 3/1959 Jatfee 75-175.5 2,919,189 12/1959 Nossen et al 75-135 3,190,750 6/1965 Staggers et al. 75-135 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner US. 01. X.R. 7s 13s, 13s 

